Accepted Manuscript

Title: Electroless Nickel Plating of Arc Discharge SynthesizedCarbon Nanotubes for Metal Matrix Composites

Author: M. Jagannatham S. Sankaran Prathap Haridoss

PII: S0169-4332(14)02416-7DOI: http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.150Reference: APSUSC 29015

To appear in: APSUSC

Received date: 29-4-2014Revised date: 9-10-2014Accepted date: 27-10-2014

Please cite this article as: M. Jagannatham, S. Sankaran, P. Haridoss, Electroless NickelPlating of Arc Discharge Synthesized Carbon Nanotubes for Metal Matrix Composites,Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.150

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Electroless Nickel Plating of Arc Discharge Synthesized Carbon Nanotubes

for Metal Matrix Composites

M. Jagannatham, S. Sankaran, Prathap Haridoss*

Department of Metallurgical and Materials Engineering,

Indian Institute of Technology Madras, Chennai-600036, India.

*Corresponding author address:

Department of Metallurgical and Materials Engineering,

Indian Institute of Technology Madras, Chennai-600036, India.

E-mail: prathap@iitm.ac.in, Phone: +91-4422574771

Abstract

Electroless nickel (EN) plating was performed on arc discharge synthesized multiwalled

carbon nanotubes (MWCNTs) for various deposition times. X-ray diffraction (XRD),

Transmission electron microscopy (TEM), and Raman spectroscopy characterization

techniques are used to identify the presence of nickel deposition on the carbon nanotubes

(CNTs) and the degree of graphitization. The results indicate that impurities are less in the

purified CNTs as compared to raw carbon soot. Increasing deposition time up to 60 minutes

increases uniform deposition of nickel throughout the length of the CNTs. However, for

deposition time longer than 60 minutes, nickel particles are seen separated from the surface

of the CNTs. Uniformly coated nickel CNTs throughout their length are potential candidates

for reinforcements in composite materials. Magnetic properties of the nickel coated CNTs,

with deposition time of 30 and 60 minutes were also evaluated. The magnetic saturation of

nickel coated CNTs with deposition time of 30 minutes is less compared to nickel coated

CNTs with deposition time of 60 minutes.

Keywords: Carbon nanotubes, EN coatings, Deposition time, TEM, Metal Matrix

Composites, Magnetic Properties

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1. Introduction

Ever since their discovery by Iijima [1], carbon nanotubes have been attractive materials for

several possible applications either in single walled carbon nanotubes (SWCNTs) or multi

walled carbon nanotubes (MWCNTs). CNTs are interesting materials due to their excellent

mechanical, electrical and thermal properties. Recent studies on mechanical properties of

CNTs through in-situ tests in SEM [2] and TEM [3] demonstrated high strength, between 20

GPa-63 GPa, and high Young’s modulus, between 590 GPa-1105 GPa. CNTs are considered

as a promising reinforcement materials in metal, polymer and ceramic matrix composite

materials for structural applications because of their attractive mechanical properties, large

aspect ratio and low density. In metal matrix composites (MMCs), especially in Al based

composites; CNTs have been used as reinforcement to enhance the mechanical properties.

Many of these MMCs are produced by powder metallurgy (PM) processes [4-12]. However,

PM process is time consuming and cost of processing the composites is high. On the other

hand, casting is a simple and cost effective process for the fabrication of composites, but

producing MMCs by casting route is difficult. Limited wetting of the CNT reinforcement by

the matrix is a major obstacle for composite production by casting. Several studies have been

conducted on metallic coatings to modify the surface characteristics of CNTs [13-26].

Metallic coatings on CNTs aim to enhance the interface adhesion between the reinforcement

and the metal matrix by improving the wetting characteristics. Metallic coatings on CNTs

also enhance the electrical conductivity in CNT reinforced polymer matrix composites. The

most common used methods for applying metallic coating on CNTs are electroless and

electrolytic plating. Of these, electroless plating is very easy and is a relatively quick process.

In general, it is necessary to sensitize and activate the substrate before electroless plating in

order to obtain good quality coatings. To the best of the knowledge of authors, Li et al. were

the first to investigate electroless nickel plating on CNTs and achieved deposition of nickel

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particles on the CNTs [13]. Ang et al. performed EN plating on CNTs using a single

activation step prior to plating [14]. Ang et al. also studied electroless nickel and copper

plating [15]. Several studies have been focused on tribological and corrosion behavior of

nickel coated CNTs [16-22]; however, very few of them are focused on the magnetic

properties of these coated CNTs [23-25]. Lin et al. studied electroless Ni deposition on CNTs

for hydrogen storage applications [26]. It is noticed that many EN coating studies are

performed at room temperature using a single experimental condition. Literature on effect of

parameters for electroless plating of Ni on CNTs, are very limited. However, it is necessary

to study the effect of EN coating parameters, in particular duration of the coating process, to

obtain enhanced coatings. The duration of the coating process has significant impact on the

throughout and cost associated with the process and hence identifying the duration that also

results in excellent coating, is important.

In the present work, MWCNTs were synthesized using the electric arc discharge method

followed by purification using different methods. The purified CNTs were activated and EN

plating was performed on the activated CNTs with various stirring times. The optimum time

for EN deposition has been determined for the bath compositions chosen. The study to

determine the optimum time for EN deposition is presented, followed by the data obtained

through the characterization of the purified and the coated CNTs using XRD, SEM, TEM,

and Raman spectroscopy. The significance of the results obtained, from the perspective of

use for MMCs, is discussed. Magnetic properties of nickel coated CNTs deposited for 30 and

60 min were measured by using vibrating sample magnetometer (VSM) and the results are

discussed.

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2. Materials and Methods

2.1. Synthesis and purification of CNTs

Carbon nanotubes were synthesized using electric arc discharge technique with a rotating

cathode. The DC power supply used provided 150 A/cm2 and 32 V. Prior to arcing, graphite

powder from the anode was mixed with nickel, in a 1:1 ratio by weight. A hole, 3 mm in

diameter was made at the center of the cross-sectional surface of anode rod and filled

completely with the graphite-nickel mixture. The graphite anode was consumed while arcing

and soot was deposited on the rotating cathode with rotation speed set at 10 rpm. The

deposited soot was collected by scraping the cathode with a metal scraper. The obtained raw

soot was crushed using mortar and pestle to obtain fine soot powder. The crushed powder

was then washed with water and treated with toluene for 5 h to remove the impurities in the

raw fine powder. Toluene treated CNTs were then heat treated in a furnace at 650 oC for 3 h

in open air atmosphere followed by acid treatment, using 38% HCl, for further purification of

the CNTs. The raw soot and purified CNTs were characterized using XRD, SEM, TEM, and

Raman spectroscopy. Further, Thermogravimetric analysis (TGA) was performed on raw soot

CNTs to determine the thermal stability of the CNTs.

2.2. Sensitization and activation of purified CNTs

Purified CNTs were sensitized using a solution that was prepared with 4 gm SnCl2 and 20 ml

of 38% HCl solution (14-15), diluted with DI water for a total volume of 200 ml. This

solution was used to sensitize 2 gm of CNTs. The solution with the MWCNTs was

ultrasonicated for 20 min in an ultrasonic bath at 25 kHz frequency and stirred magnetically

for 20 min. After stirring, the solution was filtered using a G3 sintered glass and the powder

collected was dried in a hot air oven for 2 h at 120 ºC. Sensitized MWCNTs were activated

with 0.05 gm PdCl2/4 ml 38% HCl solution, diluted with DI water for a total volume of 200

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ml (14-15).The solution with MWCNTs was ultrasonicated for 20 min in an ultrasonic bath at

25 kHz and stirred magnetically for 20 min again. The solution was filtered using a G3

sintered glass and dried.

2.3. Electroless nickel coatings on activated CNTs

EN coatings were carried out on activated MWCNTs in an electroless nickel bath (14-15) for

various deposition times of 10, 15, 20, 30, 45, 60, 75, 90, and 120 minutes at a pH value of

10.3. Table1 shows the chemical composition of the bath used for the EN experiments. The

concentrations of the chemicals were the same for all the experiments. The total volume of

the solution was 100 ml in all cases. The chemicals for the EN process were mixed in a

beaker and stirred at 85±2 °C. Stirred solution was filtered using a sintered G3 filter. After

filtration, the viscous solution containing the CNTs was dried in hot air oven at 150 °C. pH of

the solution was adjusted using NaOH buffer solution and the pH was maintained by adding

NaOH buffer solution every 5 min irrespective of deposition times. The pH was measured

using a pH meter, a Chemi Line Micro controller based pH meter; Model: CL 180 with

precision of 0.01 pH and with a precision of 0.2 °C for measuring temperature.

2.4. Characterization of electroless nickel coated CNTs

EN coated CNTs were characterized by Bruker X-Ray diffractometer with Cu Kradiation

(Wavelength is 1.5405 A°) for confirmation of the phases present in the samples. The range

of scan in XRD analysis was 2= 20° to 90° with scan rate of 0.1 degree/ sec. A Quanta 400

field emission scanning electron microscope (FE-SEM), with an accelerating voltage of 30

kV, was used to characterize the surface morphology of the samples. Nickel deposited CNTs

were characterized by using a Philips CM-12 TEM, with LaB6 filament, operated at a voltage

of 120 kV. Samples used for TEM analysis were initially dispersed in methanol with

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ultrasonication at 25 kHz frequency and then the dispersed samples were placed on a carbon

coated Cu grid. EDS attached to the TEM was used to identify the local chemical

composition of nickel coated CNT samples. Raman spectroscopy was carried out using a

Horiba yvon HR-800 UV Laser Raman Spectrometer, using a He-Ne Laser with a

wavelength of 633 nm to characterize the electroless nickel coated CNTs.

3. Results and Discussion

3.1. Characterization of as synthesized CNTs

As synthesized CNTs generally contain other forms of carbon structures along with CNTs.

Purification of as synthesized CNTs reduces the impurities in the samples, as evidenced from

TEM analysis. TEM images of raw soot and purified CNTs are shown in Fig.1 (a) and Fig.1

(b), respectively. The major impurities in synthesized CNTs are amorphous carbon, metal

catalysts, and graphene sheets. The diameter of the CNTs synthesized was observed to be in

the range of 18-22 nm, but the length of the CNTs is around a few microns. Inset of Fig.1 (b)

indicates that the CNTs are multiwalled in nature. The peak at 2 = 26.4° in XRD pattern as

shown in Fig.2 indicates the graphitic nature of the CNTs. Inter planar spacing (Inter tubular

distance) of carbon nanotubes is calculated using the Bragg’s law, 2d Sin (=

Wavelength of X-rays) and d-spacing measured is 0.337 nm. It is very close to inter tubular

spacing of CNTs measured by TEM (0.34 nm). The other peaks in the XRD pattern of the

purified CNTs correspond to impurities such as amorphous carbon and graphene sheets.

From Raman spectroscopy analysis, as shown in Fig.3, it is observed that the ratio between

defect and graphitic intensities (ID/IG) is less for purified CNTs. This confirms that defects are

less in purified CNTs whereas the synthesized raw soot contains a higher amount of defects.

Fig. 4 shows the TGA curve of as synthesized CNTs. It is observed from the TGA curve that

CNTs are thermally stable up to 720 °C in open air environment. Above 720 °C, the weight

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of the raw soot CNTs sample decreases gradually. The weight gain above 800 °C can be

attributed to oxide formation on the nickel catalyst. The oxide formation is increased with

increasing temperature above 800 °C up to 1400 °C.

3.2. Optimization of nickel coatings on CNTs

Sensitization and activation of CNTs modify the surface of CNTs to make them hydrophilic

by which CNTs can attract metallic particles on to their surface. Fig.5 (a) shows the SEM

micrograph of sensitized and activated CNTs. Fig.5 (b) shows the EDS corresponding to the

sensitized and activated CNTs. EDS of the activated CNTs confirms the presence of Sn and

Pd. If active sites are formed by sensitization and activation, then it is easy to reduce the

metal ions on the surface of the CNTs at specific temperature and pH values [13]. EN plating

has been performed after the activation of CNTs. We have also performed EN coating of

CNTs without activating them. It is noticed that CNTs have not been coated in the absence of

activation. The cause for this is that because of non-catalytic characteristics of CNTs,

metallic particles will not adhere to CNTs surface without any pretreatment prior to

metallization of CNTs [27]. Hence, it is necessary to activate CNTs for metallic coatings to

deposit on them. It is observed from the XRD pattern of EN coatings as shown in Fig.6 that

the deposition of nickel on CNTs is increased with increasing deposition time from 10 min to

60 min. For the deposition time of 10 min, nickel content is less and with the increase in

deposition time the intensity of nickel peak increases. On the other hand, the intensity of peak

corresponding to CNTs is suppressed with increasing deposition time of EN plating. Presence

of phosphorus is observed from XRD pattern of EN plated CNTs due to the use of the

reduction agent, sodium hypophosphate, which yields the phosphorous. However, it is

noticed that if the concentration of the reducing agent is low, the phosphorous content is

minimal. Since phosphorous is also co-deposited during the electroless plating of CNTs,

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formation of Ni3P phase occurs because of the reaction between Ni and P. The formation of

Ni3P is undesirable as mechanical properties may deteriorate with this formation and it is

possible to reduce the formation of Ni3P by decreasing the phosphorous co-deposition in the

coatings. However, it is noticed that there is no nickel carbide observed in the XRD patterns

for all deposition times. The absence of nickel carbide is desirable since it is brittle and is

detrimental with respect to mechanical properties. Raman spectra of the nickel coated CNTs

for various deposition times are shown in Fig.7 (a). From the Raman spectrum of the nickel

coated CNTs with a deposition time of 60 min, it is observed that the peak at 626.82 cm-1 is

very strong along with D and G peak. This peak is not as strong for other deposition times.

The peak at 626.82 cm-1 corresponds to nickel and it is increased with increasing deposition

time as shown in Fig.7 (b), which is enlarged from Fig.7 (a). TEM characterization also

supports the Raman spectroscopy results on the effect of increased coating time on the

coating formed on the CNTs. Fig. 8 (a-d) show bright field TEM images of nickel coating for

various deposition times. EDS analysis of EN plating for the deposition time of 30 min is

shown in Fig. 9. EDS spectrum shows the presence of nickel. Cu in EDS spectrum is from the

grid used and Zn is from the sample holder. TEM image of Ni coated CNTs, with a

deposition time of 10 min shows that the coating is discontinuous and hence, the deposition

time for 10 min may not be sufficient for the nickel coating on the surface of the CNTs. Lin

et al. have also reported that low deposition time is not sufficient for deposition of nickel on

CNTs [26]. In their studies, it is mentioned that deposition time of 5 min was not sufficient

for Ni deposition by electroless plating. It is observed in the present investigation that the

increase in deposition time increases the nickel deposition. TEM images of EN coatings

reveals that the nickel coating with a deposition time of 60 min is uniform and it is

continuous throughout the length of CNTs. The thickness of the coating on the surface of

CNTs is a few nanometers; as confirmed by TEM images. The thickness of the coating is

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about 15 nm for a deposition time 60 min as shown in Fig. 8 (d). Increase in the deposition

time leads to the increase in metal ion reduction resulting in enhanced deposition on the

surface of the CNTs. Metal coating on the outer surface of the CNTs is uniform if there is a

greater density of active sites. In the absence of sufficient active sites, growth of the coating

normal to the surface is greater than that lateral to the surface [13].

The mechanism of the coating process including pretreatments such as sensitization and

activation of CNTs is explained in detail by Ang et al. [15]. Similar mechanism for

electroless deposition of nickel on CNTs has been explained by Kong et al. [27]. In both

these mechanisms, metal ions are reduced to metal by reducing agents and with control of

external parameters such as pH of the solution, time and temperature maintained during the

deposition process, the metal is attached to the surface of the CNTs. Uniform coating of

metal on CNTs is required to enhance the properties of composite materials. A gap between

metal particles deposited on the surface of the CNTs weakens the interfacial bond between

the reinforcement and the matrix. Such a gap has a detrimental effect on the mechanical

properties of the composite materials. EN coating with the deposition time longer than 60

minutes have also been studied. It is observed that when the deposition time is increased

beyond 60 minutes, nickel particles are detached from the surface of the CNTs. This

detachment of Ni was observed when EN deposition was carried out for 75, 90 and 120 min.

With increasing deposition time, the thickness of nickel deposited increased on the surface of

CNTs as shown in fig.10 of TEM images. The diameter of the nickel coated CNTs measured

in TEM images, is in the range of 32 nm to 40 nm for 60 minutes deposition time. However,

the diameter of the purified CNTs is 18-22 nm. Thus it is believed that the thickness of the Ni

on the surface of CNTs in the TEM images is in the range of 14 nm to 18 nm for a deposition

time of 60 min. For deposition time less than 60 min the coating is discontinuous and for

deposition times longer than 60 min, nickel particles are seen to be detached from the CNT

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surface as shown in fig.10. The formation of nickel particles detached from the surface of

CNTs leads to the formation of other phases with metal matrix when the coated CNTs are

used as reinforcements in composites such as Ni3Al in Al based metal matrix composites.

With formation of Ni3Al, mechanical properties will deteriorate since Ni3Al phase is brittle in

nature. It is therefore observed that a deposition time of 60 minutes is optimal for EN coating

on CNTs, from the perspective of use in MMCs.

3.3. Magnetic properties of electroless nickel coatings

Coating CNTs with nickel, makes them amenable to manipulation using magnetic forces.

Therefore it is of interest to study how Ni coated CNTs respond to external magnetic fields.

Magnetic measurements for nickel coated CNTs, were performed at room temperature by

using VSM (Lakeshore VSM 7410). The two samples studied had been coated for 30 minutes

and for 60 minutes with nickel. The M-H curve obtained is shown in figure11. It is observed

from Fig.11 (a) that for the sample coated for 60 minutes, the magnetization at saturation is

16.5 emu/g, whereas it is 10.8 emu/g for the sample coated for 30 minutes. These values are

lower than that of the bulk nickel which has a magnetization value at the room temperature of

54.4 emu/g [28]. Nano sized nickel often shows reduced magnetization compared to bulk

nickel because nano scale nickel has a large percentage of surface spins from which the

disordered magnetization orientation is high compared to bulk nickel [23, 29].The graph in

figure 11b indicates a lack of hysteresis suggestive of super paramagnetic behavior as

opposed to ferromagnetic behavior, consistent with nanoscale of the Ni particles. Hu et al.

reported super paramagnetic behavior of nano iron oxide particles [30]. They reported that

iron oxide nanoparticle with a size less than 10 nm, showed super paramagnetic behavior

rather than ferromagnetic behavior. They also reported that the saturation magnetization

value decreased with reduction in size of iron oxide. It is observed from fig.11 (b) that the

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hysteresis is minimal to nonexistent for the CNTs with Ni deposition durations of 30 minutes

and as well as 60 minutes. When compared to Ni coated CNTs with a deposition time of 60

minutes, the Ni coated CNTs with a deposition time of 30 minutes show less hysteresis. This

may be because the size of deposited nickel is smaller in the case of 30 minutes deposition

time compared to 60 minutes deposition. The crystalline size of Ni deposited for 30 min and

as well as Ni deposited for 60 minutes was measured by XRD by using the Williamson-Hall

method. The crystalline sizes and are 11.2 nm and 20.4 nm respectively, consistent with their

impact on the hysteresis displayed during magnetic measurements.

4. Conclusions

Carbon nanotubes were synthesized by rotating cathode arc discharge method in open air

atmosphere and purified. Electroless coating of Ni has been performed on the CNTs for

various deposition times. Deposition of nickel using electroless deposition method increased

with increase in deposition time. However, at much larger deposition times, excess Ni is seen

to be detached from the CNTs. Enhanced metal ion reduction with increasing deposition time

is responsible for the greater deposition of nickel on surface of CNTs while completion of the

coating of the CNTs leads to excess Ni appearing detached from the CNTs. A deposition time

of 60 minutes has been determined to be the optimum from the perspective of obtaining

uniform coating of Ni on the CNTs. The CNTs with uniform coating of nickel throughout

their length, produced through this work, have potential applications as reinforcements in

composite materials, particularly MMCs. The Nickel coatings are determined to be super

paramagnetic and can be manipulated accordingly if desired.

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List of figure captions

Fig. 1. TEM images of raw soot and purified CNTs confirming the fewer defects in case of

purified CNTs when compared to raw soot CNTs

Fig. 2. XRD Pattern of Purified soot confirms the graphitic nature of CNTs (The peak

observed at 2 = 26.4̊)

Fig. 3. (a-b) Raman spectroscopy analysis of raw soot CNTs and Purified CNTs; the defect

intensity is less in purified soot compared with raw soot CNTs

Fig. 4. TGA curve of as synthesized CNTs showing that CNTs are stable up to 720 °C in

open air environments

Fig. 5. (a) SEM micrograph and 6 (b) corresponding EDS of sensitized and activated CNTs

shows the presence of Sn and Pd particles

Fig. 6. XRD pattern of EN coatings with various deposition times

Fig. 7. (a-b) Raman Spectrum of EN coated CNTs for various deposition times confirms the

increase in nickel deposition with increase in deposition time

Fig. 8. (a-d) TEM images of nickel coating for various deposition times. The nickel coating is

uniform throughout the length of CNTs for the deposition time of 60 min

Fig. 9. EDS Spectrum of electroless nickel coated CNTs for 30 min deposition time shows

the presence of nickel

Fig. 10. (a-f) TEM images of nickel coating for various deposition times. (a-b) 75 min, (c-d)

90 min, and (e-f) 120 min deposition times

Fig. 11. (a) M-H curve for nickel coated CNTs for deposition times of 30 and 60 min, (b)

enlarge of fig.11 (a)

Tables: Table1. Chemical composition of electroless bath used for nickel coating on CNTs

for various experiments

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Tables

Table1. Chemical composition of electroless bath used for nickel coating on CNTs for

various experiments

Chemical used in the electroless plating Concentration (g/l)

Nickel Sulphate 25

Nickel Chloride 25

Sodium Hypophosphate 25

Trisodium citrate 16

Lead Nitrate 1.5

CNTs used 1

Deposition times (in minutes): 10, 15, 20, 30, 45, 60,75,90,120

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Electroless Ni coatings have been performed on CNTs for various deposition times.

The deposition of nickel increased with increase in deposition time.

A deposition time of 60 min has been optimum for uniform coating of Ni on CNTs.

The CNTs with uniform coating of Ni are potential for reinforcements in composites.

Electroless nickel coatings are determined to be super paramagnetic behavior.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11